This invention relates to the controlled atmospheric storage of fresh fruits and vegetables, and specifically to a method of packaging, and a package, that controls the atmosphere surrounding the packaged fruit or vegetable product to improve retention of product freshness.
Maintaining the flavor, texture and eating qualities of fresh fruits and vegetables, and extending the shelf life of flowers (hereinafter "produce" collectively) from the time of harvest through the time of consumption is an obvious problem. The most commonly used technique has been refrigeration. Some items, such as tomatoes, bananas and citrus fruits, are routinely picked in a less-than-ripe condition and stored at reduced temperatures until they are sold. Other products, such as grapes and lettuce, are picked at maturity and refrigerated. The reduced temperature helps to retard further ripening, but only for relatively short time periods and may be detrimental to the keeping quality of the product after it is exposed to room temperature.
The maturation of produce is a complex series of biochemical and developmental changes. Among the most important processes is respiration, which generally takes place according to the equation: EQU (CH.sub.2 O).sub.n+nO.sub.2 .fwdarw.nCO.sub.2 +nH.sub.2 O+heat
wherein (CH.sub.2 O).sub.n represents a carbohydrate molecule that is oxidized as the produce respires during maturation on storage.
For each produce type there is an optimum range of concentrations of CO.sub.2 and O.sub.2 at which its respiration is retarded and quality is improved to the greatest extent. For instance, some produce benefit from relatively high levels of CO.sub.2, e.g., strawberries and mushrooms, while others such as lettuce and tomatoes, store better at lower levels of CO.sub.2.
Likewise each produce type also has its own individual respiration rate, which can be expressed as cubic centimeters of oxygen per kg/hour.
It is known that the maturation-rate of produce can be reduced by controlling the atmosphere surrounding the produce so that an optimum O.sub.2 range and relative concentrations of CO.sub.2 to O.sub.2 is maintained. For instance, U.S. Pat. No. 3,102,777 suggests storage of produce in a container in which the atmosphere is continuously replenished to maintain a higher level of carbon dioxide than that in air. U.S. Pat. No. 3,450,542 suggests packaging produce (bananas) in bags of polyethylene film that has a greater permeability to carbon dioxide than to oxygen (3.81 mm (150 mil) polyethylene); the volume of air in the package is reduced to leave relatively little oxygen and an appropriate balance between the CO.sub.2 produced and O.sub.2 consumed by the produce and the relative flows of the two gases through the film is produced and maintained for an appropriate storage period (up to about 28 days). However it is a serious disadvantage that the fixed ratios of permeability involved limit the control of the atmospheric composition.
The published paper "Controlling Atmosphere in a FreshFruit Package" by P. Veeraju and M. Karel, Modern Packaging, Vol. 40, #2 (1966) pages 169-172, 254, partly overcomes that limitation by using variable-sized panels of polyethylene or permeable parchment paper in the walls of an otherwise impermeable package to establish a controlled atmosphere, and shows experimentally-derived calculations to determine the panel sizes that are appropriate for different respiration rates of produce. However, predictable areas of panels based on known respiration rates had to be replaced by variable values calculated for individual situations, and problems were encountered with the use of film, requiring excessive areas of permeable panels (over 258 cm.sup.2 (40 in.sup.2)) or the use of paper, which is undesirably wettable.
As indicated, the most advanced known controlled atmosphere storage techniques are not entirely satisfactory. There is a need for containers for packaging produce in which the atmosphere can be predictably controlled at approximately the point required to retard the ripening process and retain product freshness, while permitting the use of panels having an area of the order of 25.8 cm.sup.2 (4 in.sup.2) or less, which can easily be so situated that they are not likely to be blocked by other containers in stacking or handling. The area and permeance required are independently and directly dependent on the weight of produce enclosed.